Piezoelectric materials respond to exposure to an electrical field by deforming in a manner that varies directly with the applied potential difference. The direction of deformation, namely an increase or a decrease in length, is dependent on the polarity of the electrical field and the arrangement of the piezoelectric material. Generally, a piezoelectric element is formed by stacking a series of discs of the piezoelectric material wherein the discs are interweaved with electrodes to enable the discs to be exposed to an electrical field.
Piezoelectric elements have the advantage of being able to sustain very high cycle frequencies and to respond nearly instantaneously to a change in the electrical field. By way of example, a piezoelectric element formed of a properly oriented stack of discs can respond to a negative polarity by expanding along a longitudinal axis of the stack of discs within microseconds. For example, a stack of discs approximately 150 mm long will expand approximately 75 .mu.m.
In a typical hydraulic control system, a hydraulic valve includes an internal spool having at either end of it a control volume. High hydraulic pressure is metered into each of the control volumes at each end of the spool. Movement of the spool is used to control the output of the hydraulic valve. Movement of the spool within the hydraulic valve is often accomplished by modulation of a pilot pressure, which is also fed into each control volume at each end of the spool. The pilot pressure is under a lower pressure than the high hydraulic pressure. Thus, movement of the spool within the hydraulic valve can be accomplished by small changes in the pilot pressure at either end of the spool. Typically, operator input is used to regulate the pilot pressure to either end of the spool in order to effect movement of the spool within the valve which in turn causes movement of, for example, a machine implement.
One disadvantage with current hydraulic valves is their response time to operator input. It would be advantageous to have a hydraulic valve that responded nearly instantaneously to operator inputs. The very rapid response time of piezoelectric materials makes them ideal candidates for incorporation into a hydraulic control valve system.
The present invention is directed to overcoming one or more of the problems set forth above.